In the field of a paint for the purpose of rust prevention such as for a steel member, a rust-preventive paint has widely been used which primarily contains zinc powder and chromic acid. This paint can keep the zinc powder stable for a long time due to the passivation behavior caused by hexavalent chromium, thus providing an excellent storage stability of liquid. In addition, a coating comprised of this paint that contains zinc powder prevents corrosion of an underlying member (substrate) because the well-known sacrificial anticorrosive function of zinc effectively works, and an excellent rust-preventive effect can thereby be obtained.
In recent years, however, manufacturers of consumer products such as automobiles and precision equipments contemplate to proceed to no use of substances that contain hexavalent chromium (referred also to as “hexavalent chromium based substances”, hereinafter). Accordingly, also in the field of rust-preventive paints, there is a strong demand for paints absolutely free from hexavalent chromium based substances.
One example of such rust-preventive paints that contain no hexavalent chromium based substance is a kind of paint in which zinc powder and a binder component are dispersed or dissolved in an organic solvent, i.e. a zinc-rich paint. Such zinc-rich paints are categorized into organic ones and inorganic ones, among which inorganic ones having organic silicon compounds as vehicles are superior in terms of durability thus being used as basecoat agents in heavy-duty anticorrosive painting such as for ships and bridges.
This type of inorganic-based zinc-rich paint for basecoat is used to usually be a thick coating of 100 μM or more, but Patent Document 1 discloses a rust-preventive paint which can form a coating having an excellent anticorrosive property even if being a thin coating of about 10 μm. Primary use of such thin coatings having high anticorrosive property is for business equipments, electrical apparatuses, automobiles, etc, and specifically for secondarily fabricated components, such as fasteners including bolts and nuts, attachments including clamps and clips, and press-molded products including plates, housings, hinges and panels. These members are required to have high accuracy in assembling and at the same time high level of strength and adhesiveness of their coatings because they may be subjected to a large shear force when being fabricated and assembled.    Patent Document 1: Japanese Patent No. 4111531